She was captured at a young age, and spent years of captivity in Calormen before returning home to Narnia.
Hwin grew up in Narnia, spending her days as a foal on the green hills. She apparently had a large extended family of whom she was quite fond. At some point during her early years, she was captured and forced to serve in the southern land of Calormen, hiding her identity as a Talking Horse for fear of suffering a worse fate.
She was given into the service of Kidrash Tarkaan, a lord in southern Calormen. He gave her to his daughter, the young Aravis Tarkheena, as a riding mare. One day, when Aravis was thirteen, she rode Hwin away from her father's estate, dismounted, and drew a knife to kill herself. Hwin tried to knock the knife from her mistress' hand, and finally broke her silence to beg Aravis not to take her own life.
Aravis' despair was caused by a marriage her father had arranged between her and Ahoshta Tarkaan, a vile and elderly man whom she had never met. When Hwin revealed that she was a Northern horse, the two agreed to escape together to Narnia, where both would be free. Aravis concocted a plan to buy them a few days' time, and a few nights later Aravis disguised herself as a man, and the two rode away undetected.After at least five days of traveling, they became aware of another horse and rider not far away. Afraid of being found out, they tried to ride away, but were stopped by a lion, and forced up beside the other horse and rider.
In the aftermath of their wild race to escape the lion, Hwin complained aloud of being "so tired". The other horse stopped beside them, revealing that he too was a Talking Horse, called Bree. His rider was a Northern boy called Shasta, and both were also escaping to Narnia, a fact that prompted the four to join forces, despite the immediate dislike that Shasta and Aravis took to one another.
Shasta revealed that the Narnian ambassadors had fled from Tashbaan, afraid for their lives, and Aravis corroborated his account by telling them that the Calormene Prince Rabadash intended to launch an attack on Archenland and Narnia, in order to force Queen Susan to marry him.
With this in mind, the four raced across the Great Desert to warn King Lune of Archenland. Once they reached the mountains, they were attacked by another lion who wounded Aravis. As Hwin and Bree were far too exhausted to go on, Shasta was sent ahead to Anvard with the message, while the others rested in the home of an Archenlander hermit.
While they were at the hermit's house, he relates to the three the great battle between Rabadash and the Northerners, which he can see from afar, and of Rabadash's defeat and capture. Shortly thereafter, they were visited by Aslan, who told them that he was the lion that had drawn them together, and had wounded Aravis.
Though afraid, Hwin welcomed Him and said, "I'd sooner be eaten by you than fed by anyone else." Aslan instead kisses her fondly, and praises her for her part in their adventures.
A few days later, Shasta returned for his companions, and took them to stay at Anvard, where Aravis and Shasta remained. Bree and Hwin traveled on to Narnia a few days later, where they remained happily for the rest of their lives.
Hwin eventually married, and frequently visited her friends at Anvard. After her death, she is known to have entered Aslan's Country.
Hwin's logical thinking and humility serve as the counter to Bree's idealism and pride. She is generally very clear-thinking and reasonable, and though she seems a bit shy at times, her advice is usually the smartest of the groups. It is she, for example, who devises the best plan for getting through Tashbaan (though it falls through for reasons beyond their control).
Her wisdom is especially noticeable when compared with the attitude of Bree. When he voices his fears that the other Talking Horses might think his rolling in the grass is silly, Hwin replies she enjoys it and doesn't care what others think. She also brushes off his embarrassment about entering Narnia with a chopped-off tail (which was part of their disguise in Tashbaan).